Egypt's Islamist President-elect Mohamed Mursi took an informal oath of office on Friday before tens of thousands of supporters in Cairo's Tahrir Square, in a slap at the generals trying to limit his power."I swear by God that I will sincerely protect the republican system and that I respect the constitution and the rule of law," Mursi said to wild cheers from the crowd, many of whom were followers of his once-banned Muslim Brotherhood."I will look after the interests of the people and protect the independence of the nation and the safety of its territory," said the bearded Mursi, in an open-necked shirt and suit.Mursi is to be sworn in officially on Saturday by the constitutional court, rather than by parliament as is usual.The court dissolved the Islamist-dominated lower house this month in a series of measures designed to ensure that the generals who took over from ousted ruler Hosni Mubarak will keep a strong grip on Egypt's affairs even after Mursi takes power."There is no power above people power," said Mursi. "Today you are the source of this power. You give this power to whoever you want and you withhold it from whoever you want."His defiant speech was a clear challenge to the army, which also says it represents the will of the people.The 60-year-old US-trained engineer addressed himself to "the Muslims and Christians of Egypt" and promised them a "civil, nationalist, constitutional state."Mursi also paid homage to a militant Egyptian cleric jailed in the United States. "I see the family of Omar Abdel-Rahman (in Tahrir)," he said. "And I see the banners of the families of those who have been jailed by the (Egyptian) military." He pledged to work for the release of the prisoners, including Abdel-Rahman.